Pumping arrangements for arrays of planar optical devices

ABSTRACT

Plural planar optical devices are simultaneously pumped by a single pumping source. Various arrangements for accomplishing such pumping are disclosed. By utilizing these arrangements, the topology and routing of integrated arrays including optical devices are simplified.

BACKGROUND OF THIS INVENTION

This invention relates to planar optical devices and, more particularly,to arrangements for pumping integrated arrays of such devices.

A variety of optical devices made in the form of planar opticalwaveguides are known. Multiple such devices can be fabricated in anintegrated fashion on a substrate to provide a microminiature componentthat is useful in optical signal processing networks.

An advantageous planar device that functions as an optical amplifier isdescribed in U.S. Pat. No. 5,119,460. The device described thereinamplifies optical signals by stimulated emission from erbium ions.Erbium ions are excited to a lasing level by coupling pumping radiationinto an active erbium-doped region of the device.

In accordance with the teachings of the aforecited patent, signal andpumping radiation are combined and then injected into the active regionof each device (see, for example, FIG. 2 of the patent). For largeintegrated arrays made on a single substrate, such device-by-devicepumping can obviously lead to complicated topologies and difficultrouting problems.

Accordingly, efforts have continued by workers skilled in the artdirected at trying to improve the structural arrangement of planaroptical amplifiers in an integrated array. In particular, these effortshave been directed at trying to devise a more effective way of pumpingthe amplifiers. It was recognized that these efforts, if successful,could lead to more compact and lower-cost integrated arrays for use inoptical signal processing networks.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, pluralplanar optical amplifiers in an integrated array are simultaneouslypumped by a single pumping source. In any case, the number of pumpingsources in the array is smaller than the number of amplifiers to bepumped. In one embodiment, the pumping arrangement comprises a singleoptical source for applying a pumping signal to a loop-configured planaroptical waveguide having branches that are disposed adjacent to and incoupling relationship with plural active waveguide regions to beexcited. In another embodiment, the output of an elongated opticalsource that is disposed parallel to an array of active waveguide regionsis coupled to the active regions via a microlens. In yet anotherembodiment, a slab waveguide whose width approximates the length of eachactive region of an array of regions is transversely disposed and incoupling relationship with respect to the array. The output of anelongated pumping source is coupled to one end of the slab waveguide,thereby to simultaneously couple the pumping signal into each activeregion of the array.

BRIEF DESCRIPTION OF THE DRAWING

A complete understanding of the present invention and of the above andother features and advantages thereof will be apparent from the detaileddescription below taken in conjunction with the accompanying drawing,not drawn to scale, in which:

FIG. 1 is a top view of a portion of a specific illustrative integratedarray, including a loop-configured waveguide, that embodies theprinciples of the present invention;

FIG. 2 is a cross-sectional representation of the FIG. 1 array, with anupper cladding layer added thereto;

FIG. 3 is a cross-sectional side view of an embodiment of the inventionin which an elongated optical source is coupled to plural active regionsvia a microlens;

FIG. 4 is a cross-sectional end view of the embodiment depicted in FIG.3;

FIG. 5 is a top view of a portion of an embodiment of the invention inwhich a transversely disposed slab waveguide is utilized to pump pluralactive regions;

FIG. 6 is a cross-sectional end view of the FIG. 5 arrangement, with anupper cladding layer added thereto;

and FIG. 7 is a cross-sectional end view of a modified version of theembodiment shown in FIGS. 5 and 6.

DETAILED DESCRIPTION

For the purpose only of providing a particular illustrative example, theplanar waveguide devices included in the integrated arrays describedbelow will be assumed to be optical amplifiers comprising erbium-dopedactive regions. It is emphasized, however, that the various arrangementsto be described are not limited to devices in which erbium-doped regionsare pumped. The inventive arrangements are generic in nature and thusare also clearly applicable to pumping a variety of other known activedevices in which emission occurs from ions other than erbium ions.

The particular erbium-doped amplifiers assumed herein are used toamplify optical signals by stimulated emission from Er³⁺ ions. As iswell known in the art, the appropriate signal wavelength for such anamplifier is about 1.55 micrometers. Various wavelengths of pumpingradiation (for example, wavelengths of 514 nanometers, 660 nanometers,810 nanometers, 980 nanometers and 1.48 micrometers) are effective toexcite Er³⁺ ions, as described in greater detail in the aforecitedpatent.

The specific illustrative integrated array partially shown in FIG. 1includes two passive waveguides 10 and 12 having active erbium-dopedtapered waveguide regions 14 and 16 (for adiabatic coupling) formed onthe respective surfaces of the waveguides 10 and 12. Illustratively,optical signals are applied to the left-hand ends of the waveguides 10and 12 from single-mode optical fibers 15 and 17, as indicated by arrows18 and 20.

In accordance with the invention, an appropriate pumping signal for theactive waveguide regions 14 and 16 is coupled thereto by aloop-configured passive waveguide 22. The Y-direction width andZ-direction height of the waveguide 22 are, for example, approximatelythe same as the corresponding dimensions of the passive waveguides 10and 12. Top and bottom branches of the loop-configured waveguide 22 arerespectively disposed closely adjacent to the active regions 14 and 16.

An appropriate pumping signal is applied to the left-hand end of thewaveguide 22 of FIG. 1, as indicated by arrow 24. Such a signal, whichis provided, for example, by a continuously operating laser, is guidedby the waveguide 22 to repetitively propagate around the loop-shapedextent thereof, as represented by arrows 26. Repetitive traversal of thepumping signal through the top and bottom branches of the waveguide 22enhances the absorption of pumping radiation by Er³⁺ ions in the activeregions 14 and 16.

As shown in FIG. 1, the loop-configured waveguide 22 is utilized tosimultaneously pump two active regions 14 and 16. By extending thewaveguide 22 in the X direction to form a longer loop, it is feasiblethereby to provide longer top and bottom branches. Portions of theselonger branches can then be coupled to additional active regions (notshown) disposed on other passive waveguides in the same integratedarray.

FIG. 2 is a cross-sectional depiction of the FIG. 1 arrangement at theline 30 as viewed in the direction of arrows 2. In FIG. 2, thewaveguides 10, 12 and 22 are shown disposed on a lower cladding layer 32which overlies a substrate 34. Also shown in FIG. 2 are the activeregions 14 and 16 and an upper cladding layer 36.

Each of the waveguides 10, 12 and 22 of FIG. 2, as well as each of theactive waveguide regions 14 and 16, is substantially surrounded by thecladding layers 32 and 36. The refractive indices of the cladding layers32 and 36 are selected to be less than the refractive indices of thewaveguides 10, 12 and 22 and also less than the refractive indices ofthe active regions 14 and 16. As a result of these refractive indexdifferences, electromagnetic radiation at the signal wavelength isguided in the waveguides 10 and 12 and in the active regions 14 and 16,and radiation at the pumping wavelength is guided in the waveguide 22and coupled to the regions 14 and 16 when the separation between thewaveguide 22 and the regions 14 and 16 is small.

Additionally, the index of refraction of each of the active regions 14and 16 of FIG. 2 is preferably designed to be somewhat greater than therefractive index of its respective underlying waveguide. In that way,the greatest possible amount of light is captured in the active regions14 and 16.

Illustratively, the waveguides 10 and 12 and the active regions 14 and16 of FIG. 2 function as single-mode waveguides for signal radiationapplied thereto from associated single-mode optical fibers. Thewaveguide 22, on the other hand, can function either as a single-mode ormulti-mode waveguide with respect to pumping radiation that is appliedto its input end.

In one specific illustrative example, the lower cladding layer 32 ofFIG. 2 is formed on an appropriately prepared planar top surface of asubstrate 34 that comprises a silicon wafer. The layer 32 is, forexample, a layer of vitreous silicon dioxide which is grown by astandard technique that involves thermal oxidation of silicon under highpressure steam. The thickness of the layer 32 should be greater thanabout ten micrometers, because optical leakage may occur atsubstantially smaller thicknesses. In one embodiment, the thickness ofthe layer 32 is, for example, approximately fifteen micrometers.

Each of the waveguides 10, 12 and 22 of FIG. 2 is made, for example,from phosphosilicate glass that is deposited on the cladding layer 32and then patterned. The phosphorus content, and thus the refractiveindex, of the deposited glass is selected to provide the desiredwaveguiding properties, in accordance with techniques well known in theart. Each of the waveguides 10, 12 and 22 has, for example, aY-direction width of about six micrometers and a Z-direction height ofalso approximately six micrometers.

Illustratively, the active regions 14 and 16 of FIG. 2 are each madefrom a silica-based glass having a relatively high concentration oferbium, such as, for example, glass having an erbium-to-silicon atomicratio of about 0.01.

An exemplary method for depositing the active regions 14 and 16 of FIG.2 is by sputtering, as described in detail in the aforecited patent. Asnoted earlier above, the refractive index of each of the active regions14 and 16 should advantageously be greater than the index of itsrespective underlying waveguide (waveguide 10 or waveguide 12).

The Z-direction thickness of each of the active regions 14 and 16 is,for example, about 1.5 micrometers. The Y-direction width of each of theregions 14 and 16 (except for the tapered ends thereof) is approximatelyfour to six micrometers. And, illustratively, the X-direction length ofeach of the active regions 14 and 16 is about three to five centimeters.

The upper cladding layer 36 shown in FIG. 2 is advantageously made ofphosphosilicate glass, exemplarily by standard low-pressure chemicalvapor deposition techniques. The Z-direction thickness of the layer 36is, for example, about ten to fifteen micrometers.

In accordance with the invention, the pumping signal that is guided torepeatedly propagate around the loop-configured waveguide 22 shown inFIGS. 1 and 2 is effective to simultaneously couple pumping energy intoboth of the active regions 14 and 16. In that way, pumping radiation isabsorbed by ions in each of these regions, thereby promoting at leastsome of the ions therein to a state that is a lasing level of atomicexcitation, as is well known in the art.

In one particular illustrative example, the Y-direction distance d (FIG.2) between the facing sidewalls of the pumping waveguide 22 and each ofthe signal waveguides 10 and 12 is approximately one micrometer or lessalong substantially the entire lengths of the waveguides 10 and 12 thatrespectively underlie the active regions 14 and 16. Such a spacingprovides effective coupling of radiation from the pumping signalpropagating in the waveguide 22 into the active regions 14 and 16.

For illustrative purposes, each of the embodiments described hereinshows active regions formed on top of their respective waveguides. Butit is also feasible to form the active regions as part of or even as theentirety of the signal-carrying waveguides. Also, the pumping waveguide22 shown in FIGS. 1 and 2 could alternatively in a multilayer structurebe formed, for example, on top of the regions to be pumped.

Another specific illustrative embodiment of the present invention inwhich plural active regions are simultaneously excited from a singlepumping source is depicted in FIG. 3. In particular, FIG. 3 shows onesuch active region 40 formed on a passive waveguide 42. As in theearlier-described arrangement of FIGS. 1 and 2, the FIG. 3 structureincludes upper and lower cladding layers 44 and 46, respectively, and asubstrate 48.

Pumping of the active region 40 and of other adjacent active regions ofFIG. 3 is accomplished by an elongated optical source 50 disposed on asupporting substrate 52. The source 50 comprises, for example, aconventional surface-emitting laser array which provides multipledownwardly projecting laser beams emanating from a longitudinal axis ofthe source 50 that is parallel to the indicated X axis. These beams areprojected onto plural active regions of the FIG. 3 arrangement by a lenscomponent 54.

Illustratively, the lens component 54 of FIG. 3 comprises a slab made,for example, of quartz. A cavity 56 is formed in the bottom surface ofthe slab to serve as a concave spherically shaped microlens. Thismicrolens is designed to project the output of the elongated opticalsource 50 onto the entire X-direction extent of the active region 40.Additionally, the microlens formed in the component 54 is designed toproject the output of the source 50 onto a relatively large area in aplane parallel to the indicated X-Y plane. In particular, this area hasa Y-direction extent that exceeds the Y-direction width of the source 50and is sufficiently wide to encompass multiple active regions includingthe active region 40. Moreover, the output of the source 50 is projectedby the lens component 54 to encompass the entire X-direction extent ofall such multiple active regions.

As shown in FIG. 3, the source 50 is spaced apart from the lenscomponent 54 (by, for example, about 300 micrometers), and the component54 is spaced apart from the top surface of the upper cladding layer 44(also by, for example, approximately 300 micrometers). Illustratively,such spacing is provided by precision-ground balls 60 through 63 thatare respectively positioned in pairs of accurately formed depressions inthe facing surfaces that are to be spaced apart.

FIG. 3 also includes a so-called resonant mirror stack 66 whichcomprises multiple alternating layers of dielectrics such as silicon andsilicon dioxide spaced apart from each other by a quarter wavelength ofthe pumping frequency. Such stacks are described, for example, in anarticle entitled "Giant Enhancement of Luminescence Intensity inEr-doped Si/SiO₂ Resonant Cavities", by E. F. Schubert et al., AppliedPhysics Letters, Vol. 61, No. 12, Sep. 21, 1992, pages 1381-1383.

The stack 66 of FIG. 3, which acts in effect as a three-dimensionalgrating, is designed to serve as a reflector for pumping radiation. Morespecifically, the stack 66 thereby improves the efficiency of thepumping operation.

Illustratively, it is also advantageous to include a reflecting layer 68on the bottom or output face of the pumping source 50 shown in FIG. 3.Such a layer, which is designed to reflect the pumping wavelength, iseffective to further enhance the efficiency of the pumping operation.

FIG. 4 is a cross-sectional depiction of the FIG. 3 arrangement at theline 70 as viewed in the direction of arrows 4. As indicated in FIG. 4,pumping light emanating from the source 50 is directed by the lenscomponent 54 to impinge upon the active region 40 as well as upon twoadditional active regions 72 and 74 which are disposed on passivewaveguides 73 and 75, respectively. The path of pumping light raysprovided by the microlens in the component 54 is approximated by arrows76.

FIG. 4 also shows the previously described mirror stack 66 thatunderlies the active region 40 and its supporting passive waveguide 42.Further, two additional identical such stacks 76 and 78 thatrespectively underlie the waveguides 73 and 75 are also depicted in FIG.4. The alternating layers that constitute the stacks 66, 76 and 78 havean X-direction length that is approximately co-extensive with the lengthof the active regions 40, 72 and 74.

FIG. 5 is a top view of a portion of another illustrative embodimentmade in accordance with the principles of the present invention. TheFIG. 5 arrangement includes three passive waveguides 79 through 81adapted to propagate optical signals. Active waveguide regions 82through 84 are respectively disposed on the passive waveguides 79through 81.

Pumping of the active regions 82 through 84 of FIG. 5 is accomplished byan elongated optical source 86 whose output is applied to the input faceof a slab waveguide 87. The X-direction width of the waveguide 87approximates the X-direction length of the active regions 82 through 84.

Illustratively, the pumping source 86 comprises a conventional elongatedarray of lasers or light-emitting diodes. In any case, the source 86provides multiple output beams each emanating from spaced-apart areascentered along a line of the source that is parallel to the indicated Xaxis. In that way, each of the active regions 82 through 84 is pumped bylight from the source 86 that propagates in the slab waveguide 87 in adirection orthogonal to the direction of signal propagation in thewaveguides 79 through 84.

The slab waveguide 87 shown in FIG. 5 is made, for example, of alight-conducting material such as phosphosilicate glass. The compositionof the waveguide 87 is selected such that it exhibits a higher index ofrefraction than the upper and lower cladding layers associated with thewaveguides 79 through 84 but lower than the refractive index of theactive regions 82 through 84. In that way, pumping light is confined topropagate in the slab waveguide 87 and a substantial portion thereof iscoupled into the active regions 82 through 84.

To enhance the efficiency of the pumping operation, it is advantageousto include a reflecting layer 88 on the end face of the slab waveguide87. The layer 88 is reflective at the pumping frequency. Also, tofurther improve the pumping efficiency, it is feasible to include asimilar reflecting layer 89 on the output face of the elongated source86.

FIG. 6 is a cross-sectional depiction of the FIG. 5 arrangement at theline 90 as viewed in the direction of arrows 6. FIG. 6 shows thecladding layers 90 and 91 that are respectively disposed above and belowthe slab waveguide 87. Other components of FIG. 6 are identified by thesame reference numerals utilized therefor for corresponding elements inFIG. 5.

In the arrangement shown in FIGS. 5 and 6, it is advantageous to form apair of resonant mirror stacks associated with each active waveguideregion. Thus, for example, stacks 92 and 93 are shown in FIG. 6respectively positioned above and below the active region 82. Thesestacks, as well as those associated with the active regions 83 and 84,extend in the indicated X direction co-extensively with the X-directionlength of the active regions 82 through 84.

Each pair of resonant mirror stacks shown in FIG. 6 is tuned to thefrequency of the signal that is designed to propagate in its associatedactive region. The stacks serve to enhance the spontaneous emissioncharacteristic of the active regions 82 through 84. In that way, thesignal-to-noise ratio of the arrangement is improved.

FIG. 7 shows an advantageous variant of the structure represented inFIGS. 5 and 6. In the FIG. 7 arrangement, the slab waveguide utilized topropagate the pumping signal is shaped to improve the coupling ofpumping energy into the active regions 82 through 84. In particular, theupper surface of the slab waveguide 94 of FIG. 7 is shaped such that thethickness of the waveguide 94 is thinned directly over the entireX-direction length of each of the active regions 82 through 84. In onespecific illustrative embodiment, the Y-direction thickness of the slabwaveguide 94 directly over each of the active regions 82 through 84 isonly about two micrometers. In that way, a substantial portion of thepumping signal is confined to propagate in an extremely close couplingrelationship with respect to the active regions.

Finally, it is to be understood that the above-described arrangementsare only illustrative of the principles of the present invention. Inaccordance with these principles, numerous modifications andalternatives may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention. For example,although emphasis herein has been directed to optical amplifiers, it isto be understood that the principles of this invention are applicable toany active optical device that requires pumping such as, for instance,lasers or radiation transformers (e.g., up-or down-converters). Also, inthe particular illustrative embodiment depicted in FIGS. 3 and 4, it isfeasible to substitute multiple distributed microlenses or holographicor prism elements for the single-microlens component 54. Furthermore, inthe exemplary embodiments shown in FIGS. 5 through 7, it is feasible tocouple pumping energy into the active regions via a slab waveguide in alongitudinal rather than in a transverse manner.

What is claimed is:
 1. Apparatus comprisingplural planar active optical devices, means including a single pumping source for simultaneously pumping said devices by coupling a pumping signal to said devices from said single pumping source, wherein each of said devices comprises an active region containing ions capable of being excited to a specified inverted level by said pumping source, means for coupling an optical signal into and out of said active regions, wherein said means for coupling an optical signal into and out of each of said active regions comprises planar waveguides respectively associated with said active regions, wherein each of said active regions comprises a waveguide section having a main longitudinal axis, a substrate having a substantially planar principal surface, a first cladding layer disposed on said principal surface, wherein said planar waveguides are disposed on said first cladding layer, and wherein said means for pumping said devices comprises a loop-configured planar waveguide having branches respectively disposed in coupling relationship to at least two of said active waveguide sections.
 2. Apparatus as in claim 1 further including a second cladding layer disposed on said first cladding layer and covering exposed surfaces of said active waveguide sections, exposed surfaces of said planar waveguides and exposed surfaces of said loop-configured waveguide.
 3. Apparatus as in claim 2 wherein the index of refraction of each of said planar waveguides and said loop-configured waveguide is greater than the index of refraction of said first and second cladding layers, and wherein the index of refraction of said active waveguide sections is greater than the index of refraction of said planar waveguides. 